The objectives of the proposal are to determine molecular mechanisms of depressed post-ischemic myocardial function and assess their functional consequences in the context of a well defined model of myofibrillar contractility. Three specific aims are proposed: 1) Investigate mechanisms of reduced Ca2+ sensitivity of isometric tension. The role of altered phosphorylation states of troponin I and/or C protein and myofilament protein degradation will be assessed. The hypothesis that depressed contraction is due to reduced Ca2+ binding to troponin C because of defects in one or more troponin subunits, disruption of cross-bridge induced Ca2+ binding, or reduced cooperative effects of strong binding cross-bridges will be tested. 2) Determine molecular mechanisms underlying the decreased rate of cross-bridge detachment as measured by Vmax. Viscoelastic effects will be measured to determine if there is an increase in internal load in stunning. Exchanges of Tn subunits and myosin light chain 2 (LC2) will determine whether changes in endogenous proteins contribute to the depression of Vmax. 3) Investigate whether stunning depresses the kinetics of cross-bridge cycling during isometric contraction by measuring the rate constant of isometric force redevelopment (Ktr). Effects on cooperative alterations in cross-bridge cycling rates will be determined, and the roles of endogenous Tn subunits and LC2 in depressed kinetics will be investigated. Insights into the molecular mechanisms and roles of decreased Ca2+ sensitivity and altered kinetics of cross-bridge cycling in contractile dysfunction of postischemic stunned myocardium will be sought.